This disclosure relates to test and measurement instruments, in particular to test and measurement instruments with inputs optimized for both time domain and frequency domain analysis.
Electronic devices can operate with signals that can be represented in multiple domains. That is, an electronic device can have signals that are customarily defined mathematically in the time domain, such as digital control signals, data, and transmitter/receiver control signals, and signals that are most often defined in the frequency domain, such as modulated RF and/or optical carriers.
For example, a frequency-hopping spread-spectrum based device can change its carrier frequency according to a pseudorandom number. This pseudorandom number can be encoded in a control signal of a transmitting device. Since the pseudorandom number changes over time, the control signal encoding the pseudo random number is typically analyzed in the time domain. However, the resulting changes to the carrier are changes in frequency that are typically analyzed in the frequency domain.
Accordingly, signals can exist within a device or system that must be analyzed in both the time domain and the frequency domain. As described above, aspects of such signals can be linked together, such as the pseudorandom number and the carrier frequency. However, test and measurement instruments are typically designed for analysis in only one domain. For example, an oscilloscope can measure signals in the time domain and a spectrum analyzer can measure signals in the frequency domain. Such measuring instruments are not time-correlated. Thus, analysis of multi-domain devices described above is difficult.
Some test and measurement instruments have some multi-domain analysis capability. For example, an oscilloscope can provide a discrete Fourier transform (DFT) function to display a frequency spectrum of an input signal. However, a DFT of a digitized time domain signal is limited by the nature of a DFT. That is, to obtain a small frequency step, i.e. a fine resolution in the frequency domain, a long time span is necessary in the time domain. Similarly, to acquire data for a wide frequency span, a high sample rate is needed. Thus, fine resolution of a modulated carrier at a higher frequency requires both a high sample rate and a long time span, requiring a large acquisition memory, which is expensive or unavailable in an oscilloscope.
Furthermore, time correlation of signals in the time domain and signals in the frequency domain can be affected by sample rate and acquisition time. For example, a higher time precision in the time domain requires a higher sample rate. However, with a given fixed memory size, the higher sample rate limits the time span and thus the size of a frequency step in the frequency domain. In other words, the frequency domain analysis precision is limited by the time domain analysis precision
Since test and measurement instruments with such multi-domain capability typically have acquisition parameters such as sample rate and record length for multiple channels linked together, simultaneous analysis in both the time domain and the frequency domain can be difficult. That is, precision in one domain can be mutually exclusive with precision in the other.